So, I finished the game and was on the hunt to get all the relics, hearing that getting all of them would enable me to unlock the final part of the tattoo and whatnot. The only one I am missing is Spider 1, and that seems to be in the cave that everyone was stored in for safety, though now that I've completed the game, there's no way to get back into the cave.

119/120 Relics, and that is the last one I need. Is there some way to squeeze back into the cave, or do I have to wait for Ubisoft to update?

I think you should try the cave because I didn't get the first relic out of the cave either and as most I didn't know about the relics in the beginning either but I finished the game and have all 120 relics

----------------------------------------------Multiplayer fixes for the patch:----------------------------------------------

General:- Fixed several crashes- Fixed bug where user was unable to jump on certain surfaces- Fixed bug where players could become invisible- Fixed bug with 3D weapon images in Decoding menu- Fixed bug where weapons could disappear after completing objectives in Co-op- Fixed bug where users could get stuck in Loadout screen in Co-op- Removed “Reloading” shout after user shoots the bow- Fixed issue with overlapping UI elements when playing split-screen- Fixed bug where split-screen users could get stuck in black screen after a cinematic- Parties will no longer be allowed to numerically unbalance games- Improved and fixed several issues with host migration- Connection degradation will now properly trigger a host migration- Fixed several bugs with Loadout menu- Fixed bug that could display a profile restriction message when trying to join a lobby- Fixed bug where users could get stuck in 3rd person in Custom games- Fixed bug with Flamethrower not doing any damage in certain circumstances- Fixed bug where “Player is on the way” tag could stick to downed players- Fixed bug where clients could remain on black screen if host left the game- Fixed bug where Fire Arrows did not do fire damage- Fixed bug where Long Distance Kill was not awarded- Fixed bug where Tag Assist was not awarded- Fixed bug where Killcam wasn’t shown in certain circumstances- Fixed bug where Survival Instinct was not cancelled properly- Fixed bug where Psych gas could get dropped in the wrong place- Fixed bug where Psych gas could affect players outside of the deployment area- Fixed bug where users could get stuck when killed by Poison gas- Fixed bug where other player’s footstep sounds sometimes did not play behind you- Fixed bug where sound could get muffled when being revived- Fixed bug where user could get de-synced if killed in mid-air

Map Editor:- Added information for Player Map playlists- Improved available space for Player map names in lobby- Fixed bug where User made maps wasn’t downloaded properly in lobby- Fixed bug that made the user stuck when igniting Firestorm nodes on certain maps

----------------------------------------------Single Player fixes for the patch:----------------------------------------------

- Fixed issue with accessing the camera and the throwing rocks.- Fixed issue with accessing weapons. The user weapon inventory will be restored as soon by entering in a safehouse or by fast travelling.- Users that have miss the Relic located in Dr. Earnhardt cave will have the relic recover.- Fixed the bug with the weapon models staying on screen / stuck on the character arm.- The leaderboard is now updating correctly even after if the user is disconnecting/reconnecting the ethernet cable- The objective is now properly updated after Jason burns the weed fields.- New options the hide the HUD are now available in the option menu.- Add support for downloadable content.- Fixed the issue when the user was becoming invincible after failing mission 'Black Gold' several times.- The Phonecall from Hurk (ULC missions) is no longer overlapping the brief of the 'Piece of the past' mission.- Fixed the game crashes happening when the user reconnect to Ubisoft servers using an account that has unlocked any MP item.- Fixed the issue when Sam was no longer in the jeep's turret after placing two bomb and being kill several time in 'Black Gold' mission.- Sam is no longer getting stuck when the user destroy an enemy car.- Fixed the buttons issue related to the Lefty Stick Controls scheme.

I think you would have to start a new file to get the last relic. what I did when I was playing through the 1st time I went for the relics only if they were nearby and not to far out of the way and I could get them without worrying about alerting pirates/privateers. also buying the maps early on helps a lot too.